Substations & distribution switchgears

The subject of this article is to discuss few most common types of MV/HV power substations based on the type of distribution switchgear, which can be seen in every distribution network. There are a number of methods of construction of substations depending on the switchgear type.

1. Open air substations

Open air substations contain separately mounted and interconnected switching equipment and components, like current transformers (CTs), voltage transformers (VTs), busbar support insulators, cable sealing ends, etc., where atmospheric air provides the main insulation path to earth.

The solid insulators MUST be designed to be capable of withstanding, for many years, all environmental conditions, e.g. rain, ice, snow, wind loading, temperature variations, pollution, lightning activity and associated switching activity to prevent dielectric breakdown to earth or between phases.

This is very important requirement and it requires relatively large clearances and, in consequence, open-air substations tend to cover large ground areas. Also, if substation is located near local communities, they’re usually not very happy about its location.

Some degree of landscaping is normally required to minimize the visual impact.

2. Metal enclosed

With metal enclosed arrangement all switching devices and associated components are enclosed in a metallic earthed structure on a per-bay basis and are cable connected.

This arrangement has the disadvantage that there is no segregation within the panel. Hence, a fault in an instrument transformer, e.g., could readily spread to encompass the switching device and panel interconnecting busbars.

Such arrangements are not generally used where high-reliability and high-availability systems are required, i.e. at bulk supply points or at primary distribution substations.

They are, however, widely used for distribution secondary substations, e.g. ring main units. Such arrangements have the advantage that they may be physically small in size and only require a fraction of the land area required by an equivalent open-air substation.

Metal-enclosed substations may be either of the outdoor type, in which case the enclosure must protect the internally connected equipment against all prevailing environmental conditions, or alternatively the substation may be enclosed within a building or low-cost weatherproof housing.

3. Metal-clad

This is a derivative of the metal-enclosed switchgear where all major components per bay are physically segregated from one another by means of earthed metalwork such that a fault in any one compartment cannot readily spread to adjacent compartments.

It is vitally important that busbars are unaffected by a fault in any particular circuit panel so that adjacent panels can safely remain in service until appropriate repairs can be executed. Typical segregation is between cable box and instrument transformers, instrument transformer chambers and circuit-breaker and circuit-breaker and busbars.

Such arrangements allow the achievement of higher reliability and higher availability systems than may be achieved with metal-enclosed concepts.

Most primary switchboards are of the metal-clad type. For economic reasons many such switchboards were originally designed for outdoor use. However, in harsh environmental climates environmental deterioration may readily occur and maintenance costs may be high.

It is now common policy for such metal-clad switchgear to be housed in appropriate weatherproofed buildings.

Figure 4 – Metal-clad switchgear type UNIGEAR, by ABB

A further modern trend is for them to be housed in transportable containers (mobile substations) such that, as systems and local load patterns change, they can readily be transferred to more appropriate locations.

Figure 5 – Mobile MV/MV substations can be easily transport and energized in case of emergency and temporary situations

4. Gas-insulated substations (GIS)

A further form of metal-clad substation that is used mainly at transmission voltages is the gas-insulated substation. With this concept SF6 provides the main dielectric medium between the primary conductor and earth.

In view of the very high dielectric strength of SF6 the dimensions can be made very small.

There are two basic concepts: The first is where each phase is physically enclosed within earthed metalwork and the other is where the three phases are enclosed within one chamber. The former has a higher degree of integrity as phase-to-phase faults cannot occur. The latter, however, provides a more economic and more compact concept.

Both arrangements are widely used, with the phase-isolated concept being used mainly at higher voltages and the three phases in one tank concept being used at the lower transmission, or high distribution, voltages, i.e. 132 kV.

Some manufacturers have, however, utilised the phase-isolated concept down to voltage levels of 33 kV, and there are a number of such installations within the UK.

Insulation considerations

Correct insulation design is vital for all substations. This applies not only for openair types and insulation-enclosed types but also for metal-enclosed and metal-clad types, which employ many insulating components.

For example with metal-clad equipment connections must be provided to connect one compartment to adjacent compartments. Such connections are usually by means of bushings. Insulation has also to be provided between phases and between phase and earth.

Typical insulating materials that may be employed are:

Atmospheric air,

Insulating oil,

Bitumous compound,

Oil-impregnated paper (OIP),

Synthetic resin bonded paper (SRBP) epoxy resin or

SF6.

The correct design of insulating interfaces between components and different insulation materials is vital for the long-term service integrity of the equipment.

Loss of Service Continuity (LSC)

When the accessibility of the switchgear compartments are known, then the consequences of opening a compartment on the operation of the installation can be predicted. Loss of Service Continuity (LSC) classification is defined by the IEC:

Generally, the rule is that if no accessible compartment is provided, then the LSC classification does not apply on that compartment.

Terms like ‘metal-clad’ or ‘metal-enclosed’ fall under classification of LSC. There are few LSC categories defined:

LSC1 – If any other switchgear functional unit than the one under intervention has to be switched off, then service is partial only: LSC1

LSC2 – If at least one set of busbars can remain live, and all other functional units can stay in service, then service is optimal: LSC2

LSC2A/B – If within a single functional unit, other(s) compartment(s) than the connection compartment is accessible, then suffix A or B can be used with classification LSC2 to distinguish whether the cables shall be dead or not when accessing this other compartment.

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Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.